The overall objectives of this project are, first, to measure directly the forces which determine the shape of adherent animal cells in culture and which govern cellular mechanical functions such as locomotion, phagocytosis, and cytokinesis, and, second, to characterize the structural roles of various cellular components, particularly the static and dynamic constituents of the cytoskeleton. We have constructed a "cell poker" (1) which allows us to measure the force required to make small indentations (less than or equal to 1 micrometer) in the surfaces of adherent cultured cells. During the current year we intend to use an improved version of this device to carry out systematic studies of the role of cytoskeletal elements in determining cell shape and "stiffness." We will also use immunofluorescence and electron microscopy to provide structural information.